It has been well known for many years to grow plants in growth substrates formed from mineral wool. Mineral wool products which are provided as a coherent block, plug, slab or mat/blanket generally include a binder, usually an organic binder, in order to provide structural integrity to the product, and such binders are commonly hydrophobic once cured. This allows the growth substrate to retain its structure during water irrigation, including use for over one year. However mineral wool products which are to be used as growth substrates also need to have the capacity to take up and hold the water which is routinely supplied by an irrigation system to the growth substrate, and re-wetting properties. Accordingly, it has been well known for some years to include a wetting agent in mineral wool products which are to be used as growth substrates.
It is important in commercial growing that the growth substrate used enables the plants to grow both rapidly and all at substantially the same rate so that when the grower reviews the plants, the greater proportion are in the most desired category. It is especially important that plants within one category are as uniform as possible so that there is less variation between the plants in the category.
One early example of a mineral wool product which can be used as a growth substrate is given by GB-A-1336426, which describes readily wettable mineral wool products suitable for use as growth substrates. To provide structure and shape, the fibres contain a binder such as a phenyl formaldehyde resin, or urea formaldehyde resin. To provide the required water-holding characteristics the product also contains a wetting agent, in an amount of up to 2% by weight based on the mineral wool. General classes of wetting agents are mentioned, such as anionic and cationic wetting agents, but non-ionic wetting agents are said to be preferred. Examples are alkyl polyglycol ethers, such as Triton CF10 and Lissapol N.
Publications, and the commercial field itself, have generally followed this approach for many years. For instance, EP-A-889175, DE-A-4208733, WO92/04824 and DE-A-4024727 all describe systems in which a wetting agent/surfactant is included in the mineral wool growth substrate, and in which the mineral wool binder is generally hydrophobic.
WO2008/009467, WO2008/009462, WO2008/009463 and WO2008/009465 all disclose the use of an ionic surfactant as a wetting agent. WO2008/009460 discloses the use of an anionic surfactant as a wetting agent. These documents disclose the use of a phenol urea formaldehyde resin as binder in conjunction with these ionic surfactants. Such a system is also used commercially.
Other types of binder than the standard phenol urea formaldehyde type have been disclosed for use in mineral wool growth substrates.
For instance, it is known to use sugar components in binder compositions used in mineral fibre growth substrates for use in growing plants. For example, WO2008/089849 discloses a mineral fibre product for hydroponic applications having a low-phenol or phenol-free binder. The binder is based on reaction products obtained by curing an aqueous solution comprising citric acid, ammonia and dextrose.
EP0631466 discloses a cultivating medium for plants consisting of bonded mineral wool, in particular rock wool, characterised in that a polysaccharide which is insoluble in cold water is provided and made to adhere to at least part of the surface of said mineral wool.
WO07129202 discloses a hydrophilic curable aqueous composition for use in a plant growth substrate wherein said curable aqueous composition is formed in a process comprising combining the following components:
a hydroxy-containing polymer,
a multi-functional crosslinking agent which is at least one selected from the group consisting of a polyacid, salt(s) thereof and an anhydride, and                a hydrophilic modifier; wherein the ratio of (a):(b) is from 95:5 to about 35:65.        
The hydrophilic modifier can be a sugar alcohol, monosaccharide, disaccharide or oligosaccharide. Examples given include glycerol, sorbitol, glucose, fructose, sucrose, maltose, lactose, glucose syrup and fructose syrup.
It is has been disclosed to use sugar components in binder compositions used in mineral wool products other than those for use as growth substrates. This is shown in WO2009/019232 which discloses the manufacture of a mineral fibre thermal insulation product wherein the binder solution comprises a reducing sugar and an acid precursor derivable from an inorganic salt and a source of nitrogen.
In the field of plant growth substrates, it has also been disclosed to create a hydrophilic binder, in order to improve the water retention properties of the binder. An example of this is EP1961291. This document discloses a method for producing water-absorbing fibre products by interconnecting fibres using a self-curing phenolic resin and under the action of a wetting agent, characterised in that a binder solution containing a self-curing phenolic resin and polyalcohol is used. It is known in the art therefore to create more hydrophilic binders using a wetting agent.
It is has been disclosed to choose a hydrophilic binder system for use in agriculture and horticulture, for instance, as a plant growth substrate. This is shown in WO2008/020073.
Whilst such systems are effective generally, there is room for improvement of the growth substrate product in various respects. It would be desirable to provide systems which show water handling capabilities, such as: improved re-saturation properties (ability of the growth substrate to take up water a second time once it has been wetted and then dried); improved water distribution properties (ability of the growth substrate to hold water at a more uniform concentration throughout the height and the volume of the growth substrate rather than concentrating at the base); and water retention. It would be desirable to provide systems which show improved seed retention and propagation, rooting-in and plant growth with a higher proportion of plants in the required selection category and with the highest uniformity between the plants.